Normal articular cartilage functions to absorb shock, to bear load and to provide articulating surfaces for diarthorodinal joints. Articular cartilage differs from other musculoskeletal tissues in that it does not have the ability to repair itself following traumatic or pathologic afflictions. Because adult articular cartilage is avascular and acellular, healing of this tissue is very difficult to achieve Bora, F. W. and Miller, G. "Joint Physiology, cartilage metabolism, and the etiology of osteoarthritis." Hand Clin. 3: 325-336, 1987!. The composition of articular cartilage varies with anatomical location on the joint surface, with age and with depth from the surface Lipshitz, H. et al., "In vitro wear of articular cartilage." J. Bone Jt. Surg., 57:527-534, 1975!.
Once the disease or trauma affects the health of articular cartilage, an inevitable degenerative process can occur Convery, F. R., Akeson, W. H., and Keown, G. H., "The repair of large osteochondral defects." Clin. Orthop. Rel. Res., 82:253-262, 1972!. During cartilage degeneration, the amount of interstitial water increases, the proteoglycan content decreases, and the aggregation of proteoglycans decreases McDevitt, C. A. and Muir, H. "Biochemical changes in the cartilage of the knee in experimental and natural osteoarthritis in the dog." J. Bone Jt. Surg. Br!, 58-B:94-101, 1976!. When the proteoglycan content decreases, cartilage becomes softer Kempson, G. E. et al., "Correlations between stiffness and the chemical constituents of cartilage on the human femoral head." Biochem. Biophys., 215: 70-77, 1970; Jurvelin, J. et al., "Softening of canine articular cartilage after immobilization of the knee joint." Clin. Orthop. Rel. Res., 207:246-252, 1986!.
The condition of cartilage can be evaluated using various methods including visual examination, mechanical probing, imaging diagnostics, and biopsies. Clinically it is very difficult to evaluate cartilage health in a non-destructive manner and most often visual observations made arthroscopically in conjunction with mechanical probing are used. Visual examination is basically a subjective, qualitative determination of the structural integrity of the surface and includes a description of the articular cartilage damage present. Numerous systems have been proposed over the years, including the Outerbridge and Noyes classification systems Noyes, F. R. and Stabler, C. L., "A system for grading articular cartilage lesions at arthroscopy." The Journal of Sports Medicine., 17:505-513, 1989; Outerbridge, R. E., J. Bone Jt. Surg. 43B:752-757, 1961!. Mechanical probing utilizes a hand-held probe like a nerve hook to subjectively evaluate the stiffness of the articular cartilage. This instrument has traditionally been easy to use in an arthroscopic setting, but the information obtained is not traceable over time. Imaging diagnostics, specifically Magnetic Resonance Imaging (MRI), can be used to diagnose internal derangements of joints. Even though its overall accuracy range is acceptable Fisher, S. P., Fox, J. M., and Del Pizzo, "Accuracy of diagnosis from magnetic resonance imaging of the knee." J. Bone Jt. Surg., 73:2-10, 1991!, its cost, lack of sensitivity for lesions of the articular cartilage Halbrecht, J. L. and Jackson, D. W., "Office arthroscopy: A diagnostic alternative." Arthroscopy, 8:320-326, 1992!, and unsuitability for some patients makes it undesirable in many cases.
Many researchers have confirmed the correlation of the cartilage stiffness with the condition of the cartilage Kempson, G. E. et al., "Correlations between stiffness and the chemical constituents of cartilage on the human femoral head." Biochem. Biophys., 215: 70-77, 1970!, and it has been shown that the compressive stiffness of the cartilage is primarily determined by proteoglycans Armstrong, C. G. and Mow, V. C. "Variations in the intrinsic mechanical properties of human articular cartilage with age, degeneration, and water content." J. Bone Jt. Surg., 64-A:88-94, 1982!. Kempson, supra, reported that the greater the proteoglycan content, the stiffer the cartilage. Indentation of cartilage has been used extensively in vitro Athanasiou, K. A. et al., "Biochemical properties of hip cartilage in experimental animal models." Clin. Orthop. Rel. Res., 316:254-266, 1995; Schenck, R. C. et al., "A biomechanical analysis of articular cartilage of the human elbow and a potential relationship to osteochondritis dissecans." Clin. Orthop. Rel. Res., 299:305-312, 1994; Hale, J. E. et al., "Indentation assessment of biphasic mechanical property deficits in size-dependent osteochondral defect repair." J. Biomechanics, 26:1319-1325, 1993; Mak, A. F. and Mow, V. C., "Biphasic indentation of articular cartilage--I. Theoretical analysis." Biomechanics, 20:703-714, 1987; Rasanen, T. and Messner, K. "Regional variations of indentation stiffness and thickness of normal rabbit knee articular cartilage." J. Biomed. Mater. Res., 31:519-524, 1996! and in situ Lyyra, T. et al., "Indentation instrument for the measurement of cartilage stiffness under arthroscopic control." Med. Eng. Phys., 17:395-399, 1995; Tkaczuk, H. "Human cartilage stiffness: In vivo studies." Clin. Orthop. Rel. Res., 206:301-312, 1986; Dashefsky, J. H., "Arthroscopic measurement of chondromalacia of patella cartilage using a microminiature pressure transducer." Arthroscopy, 3:80-85, 1987! to measure the material properties of articular cartilage including stiffness. To biomechanically evaluate the articular cartilage, in vitro biphasic and even triphasic creep indentation and stress relaxation tests have been used to determine the intrinsic mechanical properties (aggregate modulus, Poisson's ratio, permeability) of the articular cartilage Mow, V. C. et al., "Biphasic indentation of articular cartilage--II. A numerical algorithm and an experimental study." J. Biomechanics, 22:853-861, 1989; Lai, W. M. et al., "A triphasic theory for the swelling and deformation behaviors of articular cartilage." J. Biomechanical Eng., 113:245-258, 1991!. In addition in situ indentation tests have been used to map various regions of articular cartilage in several animal models and show significant variations in stiffness among the various test sites Rasanen, T. and Messner, K. "Regional variations of indentation stiffness and thickness of normal rabbit knee articular cartilage." J. Biomed Mater Res., 31:519-524, 1996!.
In the literature a few devices for the measurement of cartilage stiffness in a clinical setting have been reported e.g. Lyyra, T. et al., "Indentation instrument for the measurement of cartilage stiffness under arthroscopic control." Med. Eng. Phys., 17:395-399, 1995; Tkaczuk, H. "Human cartilage stiffness: In vivo studies." Clin. Orthop. Rel. Res., 206:301-312, 1986; Dashefsky, J. H., "Arthroscopic measurement of chondromalacia of patella cartilage using a microminiature pressure transducer." Arthroscopy, 3:80-85, 1987!. Lyyra et al. use an indentation instrument for the measurement of cartilage stiffness under arthroscopic control. Based on tests in laboratory conditions with elastomer and cadaver knee joint cartilage samples, the authors concluded that such an instrument was suitable for qualitative detection of cartilage stiffness.
The desire to test compressive mechanical properties of a material existed long before a correlation between articular cartilage stiffness and the existence of articular degenerative diseases was recognized. Many devices are known for use in material indenting which are unsuitable for use for measuring cartilage stiffness due to their design. Some of the devices such as those of U.S. Pat. No. 5,146,779 (Sugimoto), U.S. Pat. No. 4,896,339 (Fukumoto), and U.S. Pat. No. 5,067,346 (Field) are designed for use on a tabletop. Since they cannot be used arthroscopically, a sample of tissue would have to be removed from the body or the patient would have to be subjected to major invasive surgery in order to allow these devices to indent the articular cartilage. Due to the injury to the patient and the expense these procedures would necessarily entail, a nonarthroscopic design is not effective for testing the in vitro stiffness of articular cartilage.
U.S. Pat. No. 5,433,215 (Athanasiou et al.) and Tkaczuk, H. "Human cartilage stiffness: In vivo studies." Clin. Orthop. Rel. Res., 206:301-312, 1986 disclose devices useful for cartilage testing; however, these devices are larger and more awkward to use than would be desirable. These devices cannot be used arthroscopically and require the joint surfaces to be tested to be completely exposed.
In order to prevent the invasive steps and awkwardness involved in the use of the above designs to measure articular cartilage, hand-held materials testers have been designed which require no more surgery than a visual arthroscopic evaluation. These, however, suffer from a plethora of other problems. U.S. Pat. No. 4,159,640 (Leveque) describes a hand-held device which is not usable for arthroscopic surgery. Leveque's device requires a necessarily wide base suited for surface tissue measurements such as skin or the surface of muscle, but is unsuited for use within joints for measurement of articular cartilage. In addition, Leveque's device must be positioned relatively perpendicular to the material to be tested and the entire device must rest on material of similar stiffness in order to accurately measure.
U.S. Pat. No. 4,503,865 (Shishido) is primarily designed to measure differences between compressibilities. The device rolls over the material and allows measurement of changes of stiffness. The device however has no means for measuring absolute stiffness and providing an objective display of stiffness. The force the operator uses to position the device will affect the results, and this force exerted by the operator is not controlled. The device can thus be used to find hard or soft spots within a specific material, but the device cannot provide a concrete determination of whether the material is soft or hard as compared to an objective standard.
The major limitation with arthroscopic devices intended to be used for measurement of mechanical properties of materials is that they do not compensate for the indenting tip being positioned at angles other than perpendicular with the material being tested. This can either be due to natural variation in the surface of the material or to difficulty on the part of the operator to maneuver the tip to a position where the tip is perpendicular. Some devices have tried to compensate for this by forcing the material to be placed perpendicular to the indenting tip (the table-top models listed above), while others have tried to ensure that the operator can effectively know when the tip is perpendicular to the material. U.S. Pat. No. 4,364,399 (Dashefsky) discloses a probe whose compressible tip is pressed into the cartilage. Due to the shape of the end of the cannula, when the operator can push no further, the compressible portion of the probe registers the appropriate stiffness (see FIG. 3B of Dashefsky for the position for a proper reading). The probe is positioned manually and perpendicularity of the probe is subjectively determined. There is no guarantee that the operator has correctly aligned the probe for any given measurement. The manual identification process is not sufficiently accurate to allow repeatable, objective measurements. U.S. Pat. No. 4,132,224 (Randolf) also discloses a device which is positioned manually and provides no means for compensation for movement. It is clear from the description of its operation that any tilting leading to the tip not being perpendicular will result in significantly inaccurate readings due to premature touching of the forked beam of this device. U.S. Pat. No. 5,503,162 (Athanasiou et al.), U.S. Pat. No. 5,494,045 (Kiviranta et al.), and Lyyra, T. et al., "Indentation instrument for the measurement of cartilage stiffness under arthroscopic control." Med. Eng. Phys., 17:395-399, 1995 describe devices having a contact surface around the tip to aid in aligning the tip perpendicular to the material being tested (in addition to using machine controls to aid alignment in Athanasiou et al.), but such additions, although aiding the operator in positioning the tip perpendicular to the test material, do not help if the operator cannot get the tip perpendicular. In all of these devices, and others, the indenting tip must be perpendicular in order for an accurate measurement of stiffness to be made. No matter how many structures are added to these devices to try and insure perpendicularity, they will all give significantly inaccurate readings if the indenting tip cannot be aligned perpendicular to the material to be tested. In articular cartilage measurements, especially in small joints such as finger, ankle, or temporal mandibular joints, there is a high possibility that the device cannot be aligned perpendicular to the material being tested due to intervening structures such as bone, muscle, or other body parts. There is thus a need in the art for a materials tester that does not have to place its indenting tip perpendicular to the material being tested in order to provide accurate measurements.
In addition to these limitations, devices known to the art are usually unable to compensate for temperature variations during the measurement. The art makes limited reference to compensating for temperature effects although such effects can significantly impact the measurements of the device, especially when measurements are taken in situ in the body with devices calibrated outside the body.
These devices often also indent the material great distances over long periods of time. Although for many materials such indentation time and distance are not relevant, in the case of articular cartilage, long, deep indentation steps can result in significant tissue damage.
Furthermore, the operator may introduce error due to the varying amounts of force the operator uses to bring many prior art devices into contact with the material to be tested. In a table top device this is not a problem since the operator need not hold the device against the material, but may place the material on a prepared surface and allow indentation controlled by machine or computer. In many hand-held devices known to the art, however, if the operator changes the force used to depress the testing tip of the device into the material to be tested, the device will report compressibilities of different values.
Finally, none of these devices are designed to allow the portion inserted into the patient's body to be for single use only. Since a device for single use is significantly more sterile and sanitary than a reused device, such a quality is to be desired. Known devices are generally not completely watertight and thus submersible which affects their ability to be sterilized via liquid sterilization methods and also makes them vulnerable to splashes, from body fluids or otherwise, that could damage their delicate electronic components. For a device used in situ for the measurement of body tissue stiffness, survivability under surgical conditions where fluids are prevalent is a highly desired quality.
It is an object of the invention to provide a novel materials testing device which is free of the above-mentioned defects of the art.